Isoluminous additive color multispectral display

ABSTRACT

Each of a set of multispectral photographic negatives of a given scene is made by exposure to radiation from the scene primarily in a different spectral region, and each has substantially the same gamma. From each negative a positive mask is formed of which the gamma substantially equals minus one times the gamma of the negatives divided by one less than the number of negatives in the set. Each negative is subtractively combined in register with each positive mask made from the other negatives of the set. This forms a set of masked positives which correspond respectively to the negatives and in each of which density distribution is a function substantially exclusively of the hue and saturation distribution in the scene in the corresponding spectral region. The masked positives are combined additively in a color viewer to provide an improved multispectral display.

United States Patent [191 Yost. Jr.

[ 1 Mar. 19, 1974 ISOLUMINOUS ADDITIVE conon MULTISPECTRAL DISPLAY [75]Inventor: Edward F. Yost, Jr., Northport,

[73] Assignee: Spectral Data Corporation,

Hauppauge, NY.

[22] Filed: May 11, 1971 [21] Appl. No.: 142,232

[52] US. Cl 96/5, 96/17, 96/24, 40/1061, 355/37 [51] Int. CL... G03c5/08, G03b 27/04, G03b 33/06 [58] Field of Search 96/5, 17, 27 R, 2;355/37 [56] References Cited UNITED STATES PATENTS 2,244,992 6/1947Guerrero a 96/5 Primary Examiner-Norman G. Torchin AssistantExaminer-Alfonso T. Suro Pico Attorney, Agent, or FirmEliot S. Gerber[57] ABSTRACT Each of a set of multispectral photographic negatives of agiven scene is made by exposure to radiation from the scene primarily ina different spectral region, and each has substantially the same gamma.From each negative a positive mask is formed of which the gammasubstantially equals minus one times the gamma of the negatives dividedby one less than the number of negatives in the set. Each negative issubtractively combined in register with each positive mask made from theother negatives of the set. This forms a set of masked positives whichcorrespond respectively to the negatives and in each of which densitydistribution is a function substantially exclusively of the hue andsaturation distribution in the scene in the corresponding spectralregion. The masked positives are combined additively in a color viewerto provide an improved multispectral display.

NEGATIVE #l NEGATIVE #2 NEGATIVE *3 J (N-l) 9 (N-Z) 9 (N-3) of :2 i 02rt c, 2 6 o 6 Dc 9 02 i D37 D3 2 DO w w 2 6 I Q a l D 5 i I 2 3 xv 5 A:l /\1! BLACKGREY WHITE x BLACK GRE fWHLBEG I X BLACK oREY WM I T E X L06o (Enos/chi) o (Elias/en:

MASK *2 MASK #l MASK #l (M-2) (M-l) (M-ll 0? I o? I 02 W4 1 l /z a 1 l2MASK *3 MAsK *5 MASK #2 I 'B) 0 TI *2 MASKED POSITIVE *3 MASKED POSITIVE#1 Q2 MASKED VE O p- 4 (P 2) D (P 3) 8,6." 8.0,! 8.0,W r 1 DI 7 2 m h BG R GREY SCALE R OBJECT PAIENTEIJMAR I 9 I974 NEGATIVES POSITIVE MASKSWIT-H MASKS FOR PRINTING F/G. l5

MASKED POSITIVES SIIEEI 1 [If 3 N"I N-Z N-3 NEGATIVE NEGATIVE NEGATIVEMASK MASK MASK #I #2 #3 I M-I M-Z M-3 N-l /N-Z /N"3 NEGATIVE NEGATIVENEGATIVE MASK LIA-2 MASK MASK MASK L MASK MASK PI P-2 P3 I f I POSITIVEPOSITIVE POSITIVE INVENTOR. EDWARD F. YOST,JR.

BY Mv his ATTORNEYS PATENIED MR 1 9 1974 SHEET 2 OF 3 X E M T S W E E) YW3 M510 T. G AN K m N v0 W- O3 4 D 2 U11 W3 G 3 83 D D D tm2m0 vX LI m lL H E 2 w 1 l I I I IIAIO Y W2 E l. n G mm K A G C 2 N v A 6 2 l 2 6 8U1 W nnUc 2 D F rtwzwo m c l E6 R WE M E Y6 E Rm 6 MN l I I IIAIO m n mN 0 M B F 0 M \l U: WW- 6 OIB MASK #l MASK #2 MASK #3 0 FIG. 2C

MASKED POSITIVE #2 MASKED POSITIVE 3 MASKED POSITIVE l INVENTOR. EDWARDF. YOST, JR.

FIG. 2

his ATTORNEYS OBJECT PATENTED MAR 1 9 I974 SHEET 3 OF 3 FIG. 3A

FIG. 30

INVENTOR EDWARD F. YOST,JR.

MMMVKW his ATTORNEYS.

BACKGROUND OF THE INVENTION This invention relates to photography and,in particular, to novel and highly effective multispectral photographywherein a composite additive color display of substantially uniformbrightness is generated.

In multispectral photography as practiced heretofore, a given scene isphotographed in black-and-white simultaneously and from the sameperspective by a number of cameras-typically four. The four cameralenses are precisely matched as to field distortion and focal length andmay be mounted for convenience in a single housing. Each lens transmitslight in a different spectral region: for example, blue, green, red andinfrared. The selective transmission characteristics of the lenses arereadily established by the use of conventional filters.

Negatives are developed from the four photographs, and a positive ismade from each negative. The positives, or a selected number of them,for example three, are projected simultaneously by separate projectionlenses in an additive color viewer to form a composite image on aprojection screen for viewing. Separate filters are associated with theseveral projection lenses so that the composite image is in color-truecolor if the filters associated with the several projection lenses havethe same characteristics respectively as the filters associated with thecorresponding camera lenses, and false color if they have not.

Multispectral photography as briefly explained above is a powerfulremote-sensing technique facilitating the collection of a great deal ofinformation from aircraft or satellites regarding vegetation and soilconditions, mineral deposits, water pollution and a multitude of otherphenomena. However, where the illumination of a scene is highly variableor where the brightness of the scene is very great as compared to thedifferences in hue and saturation, the detection of spectral differencesbecomes relatively difficult.

SUMMARY OF THE INVENTION An object of the invention is to remedy theproblem outlined above and, in particular, to provide for multispectralphotography wherein the effects of brightness are subdued and subtledifferences in hue and saturation are greatly enhanced.

The foregoing and other objects are attained by a special preparationfor additive color viewing of a set of multispectral photographicnegatives of a given scene wherein each negative is made by exposure toradiation from the scene primarily in a different spectral region andhas substantially the same gamma. In accordance with the invention, apositive mask is formed from each of the negatives. The gamma of eachpositive mask substantially equals minus one times the gamma of thenegatives divided by one less than the number of negatives in the set.Each negative is subtractively combined in register with each positivemask made from the other negatives of the set. This forms a set ofmasked positives which correspond respectively to the negatives and inwhich density distribution is a function substantially exclusively ofthe hue and saturation distribution in the scene in the correspondingspectral region. The masked positives are used to form an additive colorcomposite image wherein brightness is substantially constant anddifferences in hue and saturation are accentuated.

BRIEF DESCRIPTION OF THE DRAWING An understanding of other aspects ofthe invention can be gained from a consideration of the followingdetailed description of the preferred embodiments thereof, inconjunction with the appended figures of the drawing, wherein:

FIG. 1A is a schematic view showing the preparation of positive masks inaccordance with the invention from a set of corresponding multispectralphotographic negatives;

FIG. 1B is a schematic view showing the preparation in accordance withthe invention of a set of masked positives respectively corresponding tothe negatives by subtractively combining each negative in register witheach positive mask made from the other negatives of the set;

FIG. 2A is a triple graph showing, for each of the neg atives of FIGS.1A and 1B, the characteristic curves and the positions on the curves ofa hypothetical object in the photographed scene;

FIGS. 23 and 2C are multiple graphs showing, for each of the positivemasks of FIGS. 1A and 1B, halfgamma curves and the positions on thecurves of the hypothetical object;

FIG. 2D is a triple graph showing the (constant) density of allcolorless objects, regardless of brightness, on three masked positivesof FIG. 1B and also showing the density of the hypothetical object asrecorded on the three masked positives;

FIG. 2 is a diagram showing the position of the hypothetical object incolor space as it appears when the masked positives are projectedsimultaneously and additively to form a composite image for viewing, theprojection filters being blue, green and red in the case of maskedpositives l, 2 and 3, respectively;

FIG. 3A is a top plan view of apparatus for registering and exposing thepositive masks and masked positives made in accordance with theinvention;

FIG. 3B is a fragmentary sectional view taken along the lines 3B-3B ofFIG. 3A and looking in the direction of the arrows; and

FIG. 3B is a fragmentary sectional view taken along the lines 3B-3B ofFIG. 3A and looking in the direction of the arrows; and

FIGS. 3C and 3D are schematic side and end elevational views, partly insection, of the apparatus of FIG. 3A

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A shows a step in thepreparation for additive color viewing of a set of multispectralphotographic negatives N-l, N-2 and N-3 of a given scene. In accordancewith usual multispectral techniques, each negative is made by exposureto radiation from the scene primarily in a different spectral region.For example, the negative N-l may be made by exposure to radiation fromthe scene primarily in the blue region of the spectrum; the negative N2may be made by exposure to radiation from the scene primarily in thegreen region of the spectrum; and the negative N-3 may be made byexposure to radiation from the scene primarily in the red region of thespectrum. The set as photographed may have any number of negatives init, and, in particular, it often has at least a fourth negative (notshown), made by exposure to radiation from the scene in the infrared.Each negative of the set has the same characteristic curve, at least inthe straight-line portion, and therefore the same gamma, which isdefined as the slope of the straight-line portion of the characteristiccurve. From the several negatives, positive masks Ml, M2 and M3 arerespectively formed, preferably by contact printing, each positive maskhaving the same characteristic curve and the same gamma as every otherpositive mask, but the gamma of the positive masks being different fromthe gamma of the negatives and related to the gamma of the negatives bya formula that depends on the number of negatives in the set.

As FIGS. 2A, 2B and 2C show, the gamma of each negative can be regardedas 3 (since, ify 'yx, where x and y are variables and 'y is a constant,dy/dx =7), and the gamma of each positive mask can be regarded as y/2(since, ify yx/2 plus a constant, dy/dx 'y/2). In general, the gamma ofeach positive mask is chosen so that it substantially equals minus onetimes the gamma of the negatives divided by one less than the number ofnegatives in the set. The size of the set in question equals the numberof photographs to be simultaneously projected, and not necessarily thenumber of photographs that were originally taken simultaneously. In theparticular embodiment illustrated in the drawings, the number ofnegatives in the set to be projected is three, one less than that numberis two, and the gamma of the masks is therefor minus one times onehalfthe gamma of the negatives. It will be shown below that the invention isbroad enough to be used regardless of the number of negatives in theset.

As FIG. 1B shows, each negative N-l, N-2 and N3 is subtractivelycombined in register with each positive mask made from the othernegatives of the set. That is, the negative is subtractively combinedwith the masks M2 and M3; the negative N-2 is subtractively combinedwith the masks M1 and M3; and the negative N3 is subtractively combinedwith the masks M1 and M2. This forms a set of masked positives P-l, P-2and P-3 corresponding, respectively, to the negatives N-l, N-2 and N3.In the masked positives, density distribution is' a functionsubstantially exclusively of the hue and saturation distribution in thescene in the corresponding spectral region, since the gamma of eachnegative equals in absolute value the sum of the gammas of the positivemasks with which it is subtractively combined and brightness differencesare substantially cancelled.

FIG. 2A shows the characteristic curves of the negatives N-l, N-2 andN-3. Each of the triple graphs plots density on the ordinate or y axisas a function of the logarithm of the intensity (in ergs per squarecentimenter) on the abscissa or x axis. On the x axis, log intensitiescorresponding to black, grey and white are indicated, and, on the yaxis, corresponding densities D, D" and D are indicated, subscripts l, 2and 3 being used in the case of the negatives N-l, N2 and N-3,respectively. Also on the x axis of each graph, the log intensityassociated with a hypothetical object O in the scene is indicated, and,on the y axis, a corresponding density D". Again, subscripts l, 2 and 3are used in the case of the negatives N-l, N-2 and N-3, respectively. Onthe arbitrary density scale chosen, the density associated with'theobject O is 1 /2 in the case of negative N-l, 2% in the case of negativeN- 2, and 2 in the case of negative N-3. The hypothetical object O inthe scene is thus between black" and grey on negative N-l, between grey"and white on negative N-2 and at grey on negative N-3. These positionsare a function of the filters associated with the lenses used in formingthe images on the negatives and of the spectral characteristics of theobject.

FIGS. 2B and 2C show the characteristic curves of the positive masks M2and M3 subtractively combined in register with the negative N-l; of thepositive masks M1 and M3 subtractively combined in register with thenegative N-2; and of the positive masks M1 and M2 subtractively combinedin register with the negative N3.

Consider first the subtractive combination of the negative N-l, the maskM2 and the mask M3. As noted above, the object O is recorded on thecharacteristic curve of the negative Nl between black and grey" and thecorresponding density D, is 1 /2. In the case of the mask M2, the objectO is at the same position on the x axis as it is on the x axis in thecase of the negative N2 from which the mask M2 is made: i.e., betweengrey and white. Because the characteristic curve has a negative ratherthan a positive slope, and because the gamma is one-half the gamma ofthe negative N-2, the corresponding object density will, in general, bedifferent. In this case, the density D is three-fourths. In the case ofthe mask M3, the object O is at the same position on the x axis as it ison the x axis in the case of the negative N- 3 from which the mask M3 ismade: i.e., at grey. The corresponding density D is one.

As FIG. 2D shows, the density corresponding to the object O is D," A,which is the sum of D /z, contributed by the negative N-l; D contributedby the mask M2; and D I, contributed by the mask M3.

A similar analysis shows that the object has a density D 4% on themasked positive P-2 abd a density D 4 on the masked positive P3.

FIG. 2D also shows for the masked positive P-l a graph representing thedensity of all colorless objects, regardless of brightness. A colorlessobject has the same x-coordinate on each of the three negatives andtherefore on each of the positive masks. It is easy to see frominspection of the graphs that the density sum for all such objects onall of the masked positives (or the arbitrary scale chosen) is 4. Forexample, for black," grey, and white, respectively, we have 1 2 (1.5)=4; 2+2(l)=4; and 3+2( /z)=4.

FIG. 2 shows the position in color space of the object 0 upon projectionof the masked positives P-l, P-2 and P-3 simultaneously and additivelythrough blue, green and red filters, respectively. Since the density D,"is 3%, which is less than the density D,'"""' (the density for allcolorless objects), the blue filter passes more light than it would passif the object 0 were at grey in masked positive P-l. The circled Xrepresenting the position of the object O in color space is thusdisplaced towards the blue vertex of the color space diagram. Thedensity D is 4%, which is more than the density D The green filter thustransmits less light corresponding to the object 0 than it would if theobject were at grey in masked positive P-2, and the position of theobject is displaced, relative to the black, grey and white point W, in adirection away from the green vertex of the diagram. In the case of thepositive P-3,

the density D is 4, which is the same as the density D corresponding toall colorless objects, and therefore the red filter transmits the sameamount of light corresponding to the object O as it would transmit forany object of neutral density. The position of the object O in thediagram of FIG. 2E is thus displaced neither towards nor away from thered vertex. In the additive color presentation the object is thusperceived as bluish-magenta. This is the true color of the object if andonly if the projection filters are respectively the same as the filtersused in taking the photographs. Otherwise the object is represented infalse color.

FIG. 3A shows in top plan view apparatus for registering and exposingthe positive masks M1, M2 and M-3 and the masked positives P-l, P-2 andP-3 for printing. A strip of film is transported between spools 12 and14 over a glass printing platen l6. Registration pins 18 in the platen16 extend through holes 20 in the film 10. The holes 20 are of largerdiameter than the pins 18. The pins 18 cooperate with holes 22 in a mask24. In order to hold the set of four negatives securely, without motion,another set of registering pins 26 is used. This maintains the principalpoints and, in fact, all images on the set of four multispectralphotographs in the same relative location during the printing process.

Unexposed film 24 is pre-punched by a conventional punch (not shown) andregistered on the pins 18 for the individual frames. The film is exposedusing the light arrangements shown in FIGS. 3C and 3D. Lamps 30 can becontrolled in intensity by continuously variable neutral-density filmstrips 32, 34. After exposure, the positive masks are developed withoutmoving the negative. The masks are then placed on the other negativeswith which they are associated as shown in FIG.

1B and in FIGS. 2A, 2B and 2C, and the masked positives Pl, P-2 and P-3for projection are produced by contact printing. As a guide in exposure,a frosted glass plate 36 can be slid in and out beneath the filmprinting platen 16.

Thus there are provided in accordance with the invention novel andhighly effective photographs for additive color display and a method andapparatus facilitating preparation of the photographs. Manymodifications within the spirit and scope of the invention of therepresentative embodiments described herein will readily occur to thoseskilled in the art upon reading the present disclosure. For example, itis possible to use the technique with two negatives, three negatives,four negatives or more, provided only that the masks are constructed sothat the gammas substantially cancel brightness differences. In the caseof a two-negative constant brightness display, the masks are made atunity gamma, since the gamma of either negative is then matched by thegamma of the mask made from the other negative. In the case illustratedin the drawings, in which three negatives are used, each mask is atonehalf gamma, since gamma of any negative is equal (except for itssign) to the sum of the gammas of the masks made from the othernegatives. Similarly, in the case where four negatives are used, themasks must be made at one-third gamma, since the gamma of any negativewould then be equal (except for sign) to the sum of the gammas of themasks made from the other three negatives.

Accordingly, the invention is to be construed as including all of theembodiments thereof within the scope of theappended claims.

I claim:

1. A method for constructing an isoluminous multispectral display from aset of masked positives of a given scene, wherein the densitydistribution in each masked positive is the function substantiallyexclusively of the hue and saturation distribution of said scene in adifferent spectral region, consisting of:

a. exposing the black-and-white negative film of the same scene using aset of not less than three different colored filters, to obtain a set ofseparate negatives with approximately the same gamma;

b. making a positive mask from each negative in the set by printing andprocessing the positive film to a gamma substantially equal minus onetimes the gamma of the other negatives ivided by one less then thenumber of negatives in the set;

c. combining subtractively and in register each negative with thepositive mask from each of the other negatives in the set, printingthese combinations onto positive black-and-white film and processing toform a set of masked positives; and

d. additively combining the set of masked positives in the color viewer.

2. The method of claim 1 wherein the masked positives of said scene arecombined additively by means of a projector.

2. The method of claim 1 wherein the masked positives of said scene arecombined additively by means of a projector.